The world supply of light crudes and those having only minor concentrations of sulfur and contaminant metals is quickly decreasing. Crude oils available on the open market today often contain high percentages of sulfur-bearing compounds and significant amounts of metal-containing organic compounds. These crudes are, in comparison to those used in the past, of a higher average molecular weight. In spite of the change in crude oil feedstocks, the products into which crude oils are made, e.g., gasoline, fuel oil, diesel, etc., remain substantially the same. Consequently, additional upgrading techniques are necessary to make optimum use of available crude oils.
Typical front-end refinery processing steps include a desalting step, which removes salt and other water soluble compounds; an atmospheric pressure distillation step, which separates lower boiling hydrocarbon components and produces a heavy atmospheric residual oil ("HAR"); and a vacuum distillation step which produces a stream of middle boiling distillates and a heavy vacuum residual oil ("HVR").
Heavy oils are not, in a typical refinery economic scheme, products which are of themselves desirable end products. They typically must be converted into some other form before they become valuable. These heavy oils, particularly the vacuum residual oils, are particularly unsuitable for conventional processing techniques such as thermal or catalytic cracking and therefore require an alternative upgrading. The oils are complex mixtures containing three general fractions: so-called "saturates", "aromatics", and "polars". Of the factors which adversely influence the quality of the heavy oils, the most noteworthy are the coke-forming tendency, metals content (particularly vanadium and nickel), nitrogen content and "polar" content. Each of these factors typically becomes more of a problem with increasing molecular weight.
One desirable upgrading technique would be to separate the heavy oil into two portions: one fraction being of high molecular weight and low quality, and the other, a desirable fraction of low molecular weight and high quality.
It has now been found that it is possible to upgrade heavy oils by mixing them with one or more solvents and applying the resulting mixture to an ultrafiltration membrane at high pressures to produce a permeate which is low in coke-forming tendencies and metal-containing compounds.